Many engine driven vehicles use an engine driven fan to provide cooling air flow across a radiator or heat transfer device to remove excess heat from the engine cooling fluid. The fan drive mechanism often has a fluid drive medium disposed between the input and output members of the clutch to control the fan speed relative to the speed of the engine. A viscous fluid, such as silicone, is generally used in these clutch mechanism. The torque capacity of the clutch is in part determined by the amount of viscous fluid filling the working chamber of the clutch. The amount of working fluid admitted to the working chamber is generally controlled by a thermostatic valve mechanism. The rotary speed of the input member of the clutch is equal to or directly proportional to the speed of the engine. As the engine cooling medium is elevated in temperature, the output member of the clutch, and therefore the fan, is increased in speed to increase the cooling air flow through the heat transfer device. These types of viscous fluid clutches are thermostatically controlled and require mechanisms for the admission and removal of the viscous fluid from the torque transfer area between the input and output members of the clutch. While these viscous clutches have performed adequately, there is a desire for more accurate fan speed control.
To provide a more controllable viscous fan clutch, it has been proposed that a magnetorheological fluid (MRF) be introduced between the input and output members of the clutch. The viscosity of the MRF can be controlled through the introduction of a magnetic field. As the viscosity of the MRF is increased, the torque transfer properties of the fluid are increased. Since the magnetic field can be controlled in intensity by a conventional electronic engine management system, the speed of the fan can be established more accurately for a given engine operating condition. Thus, a magnetorheological fluid clutch (MRC) will improve the efficiency of the engine cooling system.